Measles cases are continuing to climb in the U.S., reaching 880 this year as of May 17, according to the Centers for Disease Control and Prevention.Measles was declared eliminated in the U.S. in 2000, after widespread childhood immunization programs helped curb new cases by 99%. But driven by a few large outbreaks among communities with low vaccination rates, primarily in New York and Washington, the case count so far in 2019 has surpassed annual totals dating back to 1994, according to the CDC.Measles cases have been confirmed in 24 states, the agency says, with ongoing outbreaks in Michigan, Georgia, Maryland, Pennsylvania and Washington, as well as New York City, New York's Rockland County and California's Butte, Los Angeles and Sacramento counties.Public health officials say several outbreaks this year appear to have started with international travelers who got measles overseas and then came into contact with groups of unvaccinated people in the U.S."When measles gets into communities with pockets of unvaccinated people, outbreaks are more likely to occur," the CDC says. "These communities make it difficult to control the spread of the disease. And these communities make us vulnerable to having the virus re-establish itself in our country."The New York outbreaks, for example, have been concentrated largely among Orthodox Jewish communities and have sickened more than 700 people since late 2018. In Rockland County, about 81% of infected people had not been vaccinated for measles, while more than half of the total patients were 6 or younger.In Washington – where a 72-case outbreak centered in Clark County spurred the passage of a new law tightening vaccine exemptions this year – six new measles cases have been reported this month. Local officials believe the recent outbreak can be traced to a "common exposure" to measles at Seattle-Tacoma International Airport in late April."Health officials have no reason to believe that there is currently an increased risk of getting measles by visiting the airport," according to a release from the local health department. "Most people in our area have immunity to the measles through vaccination, so the risk to the general public is low."Health officials in Chicago, meanwhile, also confirmed one measles case in an international traveler on Friday. The infected person arrived at O'Hare International Airport last week and had been to several countries experiencing measles outbreaks, the Chicago Tribune reported.That measles case is the first identified in Chicago this year, although officials had previously reported one case in Cook County and six cases elsewhere in the state."Due to ongoing measles outbreaks in the United States and the high volume of international travelers, the risk of measles transmission in Chicago and throughout the country remains high," a release from the Chicago Department of Public Health says.Earlier this month, researchers from the University of Texas at Austin and Johns Hopkins University identified 25 counties at high risk of a measles outbreak, based on vaccination rates and international travel from countries experiencing outbreaks. Cook County ranked as the county most vulnerable to a measles outbreak in the nation.The CDC says travel to countries experiencing large outbreaks, such as Israel, Ukraine and the Philippines, poses risk of measles infection for people who haven't been fully vaccinated.Worldwide, measles cases soared 300% in the first three months of 2019, compared with the first three months of 2018, according to preliminary data from the World Health Organization. In 2017, measles is estimated to have caused nearly 110,000 deaths globally."All member states in the six World Health Organization regions have adopted goals to eliminate measles by the year 2020," the CDC says. "Once every country eliminates a disease, health officials consider the disease 'eradicated' from the world."